This invention relates to detergent compositions in the form of tablets for use in fabric washing.
Detergent compositions in tablet form have been described in, for example, GB 911204 (Unilever), U.S. Pat. No. 3,953,350 (Kao), JP 60-015500A (Lion), and EP-A-711827 (Unilever) and are sold commercially in Spain. Tablets have several advantages over powdered products: they do not require measuring and are thus easier to handle and dispense into the washload, and they are more compact, hence facilitating more economical storage.
Such tablets are generally made by compressing or compacting a quantity of detergent composition in particulate form. It is desirable that tablets should have adequate mechanical strength when dry, before use, yet disintegrate and disperse/dissolve quickly when added to wash water. It has not proved simple to achieve both properties simultaneously. As more pressure is used when a tablet is compacted, so the tablet density and strength rise, but the speed of disintegration/dissolution when the tablet comes into contact with wash water goes down.
During the period from about 1960 to 1970 there was considerable research activity in connection with tablets for use in fabric washing. A number of patents were published by major detergent manufacturers. Detergent tablets were sold commercially in USA and some European countries.
However, tablets disappeared from the market place in nearly all countries (Spain is the apparent exception) even though tablets have apparent advantages and have become known as a product form for machine dishwashing compositions which are characterised by a low content of organic surfactant.
U.S. Pat. No. 3,081,267 (Procter and Gamble) taught that the force, and hence pressure, applied when compacting a composition into tablets should be limited, or else the tablets would take too long to dissolve.
The compression pressure used in the Examples of this document was from 180 to 300 psi (approximately 1.2 to 2.1 MPa). Elsewhere in the document it is proposed that the pressure should not exceed 350 psi (approximately 2.5 MPa) to avoid slow disintegration encountered with higher pressures.
A number of proposals have been put forward as ways to improve the compromise between conflicting desiderata, but there still remains a desire to improve tablet strength without loss of speed of disintegration and vice versa.
Some documents have proposed surface treatments or coatings to enhance tablet strength. For instance U.S. Pat. No. 3,451,928 (Colgate) stated that the problem of strength versus speed of dissolution remained unsolved, and proposed a treatment of spraying on water, followed by flash heating.
U.S. Pat. No. 3,324,038 (Procter) proposed the application of a coating containing urea.
It is known to include materials whose function is to enhance disintegration of tablets when placed in wash water. Some tablets which are sold commercially incorporate urea for this purpose. Urea has a very high solubility in water exceeding 100 gms per 100 ml water at 20xc2x0 C. EP-A-711827 teaches the use of sodium citrate for the same purpose.
Detergent compositions, including tablet compositions, frequently contain a mixture of anionic and nonionic organic surfactants. It is often desirable to include both of these types of surfactant, for performance of the composition when washing fabrics.
We have now found that by using certain ingredients and formulation ranges for a tablet composition, it is possible to compact a tablet using a pressure somewhat higher than has frequently been described in prior proposals, and obtain tablets with improved strength which disintegrate and dissolve with satisfactory rapidity.
At the same time it is possible to incorporate materials which are desired to give good washing performance, and it is possible to formulate component ingredients of the tablet so that they are satisfactory in handling during tablet manufacture.
Broadly, the present invention provides a process for the manufacture of a detergent tablet which comprises mixing
(i) particles which contain non-soap organic surfactant and other materials with
(ii) material which is other than soap or organic surfactant and which has a solubility in water of at least 10 gm/liter at 20xc2x0 C., followed by compacting the mixture into a tablet or a region of a tablet using an applied pressure in a range from 3.0 to 35 MPa.
In a second aspect, the invention provides a tablet obtainable by such a process.
The particles (i) may provide from 25 to 85% of the mixture, but preferably from 30 to 65% by weight of the mixture, and in certain forms of the invention from 41 to 53, 56 or 60% by weight of the mixture.
In this invention, we have found it desirable to incorporate most (if not all) of the surfactants in the particles (i) which constitute a substantial part but by no means all the composition of a tablet. The organic surfactant in these particles desirably provides a substantial part, but again by no means all, of their weight. Preferably the particles also contain detergency builder.
Secondly, it is desirable that the surfactant is a mixture of non-soap anionic and nonionic detergent surfactants (preferably accompanied by soap) where both are present in significant amounts, but anionic is in the majority.
In certain forms of this invention, the mixture contains from 30 to 65% by weight of particles (i) which contain from 25 to 80% by weight (of these particles) of water-soluble or water-insoluble detergency builder and from 20 to 50% by weight (of these particles) of non-soap organic surfactant. This surfactant may be anionic and nonionic surfactants in a ratio from 5:1 to 1.5:1.
We have found that two different measures of tablet strength are relevant to properties observed by a consumer. Force to cause fracture is a direct assessment of strength and indicates the tablets"" resistance to breakage when handled by a consumer at the time of use. The amount of energy (or mechanical work) put in prior to fracture is a measure of table deformability and is relevant to the tablets"" resistance to breakage during transport.
Both properties are relevant to consumers"" perception of tablets: consumers will want tablets to be strong enough to handle, to reach them intact, and to disintegrate quickly and fully at the time of use.
We believe that concentrating most or all of the surfactant into surfactant-rich particles, and using a substantial proportion of anionic surfactant is beneficial in providing tablets which have both strength and elasticity, while allowing the remainder of the tablet composition to contain a substantial proportion of water-soluble material which assists disintegration of the tablets at the time of use.
It is not necessary to include surfactant as a binder material in the part of the composition outside the surfactant-rich particles. Excluding it from this part of the composition is advantageous, to avoid interference with the prompt dissolution of this part of the composition.
So, it is preferred that the weight of the non-soap anionic surfactant in the particles (i) is at least 1.7 times the weight of the nonionic surfactant in them. More preferably, this weight ratio of anionic surfactant to nonionic surfactant lies in a range from 2:1 up to 5:1, and more preferably from 2:1 to 4:1. Preferably these particles contain at least 80% by weight better at least 90% or even 95% of all the organic surfactant (including any soap) in the tablet or region.
The material (ii) which is present in the mixture, externally to the surfactant-rich particles, preferably comprises from 15 to 40% (better 16 to 35%) by weight of the mixture of one or more materials selected from
compounds with a water-solubility exceeding 50 grams per 100 grams water;
sodium tripolyphosphate containing at least 50% of its own weight of the phase I anhydrous form, and preferably partially hydrated so as to contain water of hydration in an amount which is at least 1% by weight of the sodium tripolyphosphate;
mixtures thereof.
It is strongly preferred that the water-soluble material (ii) which is present in the composition, externally to the surfactant-rich particles (i) is present as particles which are substantially free of surfactant, i.e. contain no more than 5% of their own weight of organic surfactant.
In certain preferred forms of this invention the mixture contains
(i) from 35 to 60 wt % (and probably from 41 to 53, 56 or 60 wt %) of particles (i) which contain non-soap anionic surfactant, nonionic surfactant and water-soluble or insoluble detergency builder,
(ii) from 15 to 40 wt % (and probably from 16 or 17 to 35 wt %) of material in the form of particles (ii) which are substantially free of surfactant, i.e. contain at least 95% of their own weight of water soluble material but contain no more than 5% of their own weight of organic surfactant, and
(iii) from 0 to 50 wt % of further particulate ingredients,
wherein the first said particles (i) contain at least 20% preferably at least 24% of their own weight of non-soap surfactant and the weight of anionic surfactant therein is from 1.5 to 5 times the weight of nonionic surfactant therein.
The particles (i) may be such as to be defined by reference to a test procedure described below. In such forms of the invention, the particles (i) contain non-soap anionic surfactant, nonionic surfactant, preferably soap and other water-soluble ingredients, wherein the particles (i) contain at least 20 wt % in total of the anionic and nonionic surfactants and a test tablet consisting of the said non-soap anionic surfactant, nonionic surfactant, and any soap in the same proportions, together with 15% by weight moisture has a breaking strength as herein defined of at least 0.04 MPa and a modulus as herein defined of not more than 10 MPa preferably not more than 8 MPa.
In a further aspect, this invention provides the use of a process as defined earlier to provide improvements in tablet strength and elasticity versus speed of disintegration.
In certain forms of the invention the mixture of the particles (i) and material (ii) provides (at least) a surface layer of the tablet and the step of compaction is carried out using a press with a mould consisting of a pluarlity of mould parts, some of which are relatively moveable, at least one of the mould parts bearing an elastomeric layer on a surface area which contacts the mixture.
Preferably such a layer has thickness of at least 0.3 mm, better 0.5 mm, even better over 1 mm.
We have found that by using mould parts which have such an elastomer layer, the penetration of water into the tablets on immersion is increased, thereby accelerating distintegration/dissolution of the tablets at the time of use.
As stated above, the pressure applied to bring about compaction into a tablet lies in a range from 3 to 35 MPa. Desirably, especially when mould parts carry an elastomer layer, the pressure is at least 4.0 or 4.5 MPa. A range up to 18, 20 or 25 MPa is generally suitable and the range may be narrower, e.g. up to 12 or 15 MPa.
A tablet of the invention may be either homogeneous or heterogeneous. In the present specification, the term xe2x80x9chomogeneousxe2x80x9d is used to mean a tablet produced by compaction of a single particulate composition, but does not imply that all the particles of that composition will necessarily be of identical composition. The term xe2x80x9cheterogeneousxe2x80x9d is used to mean a tablet consisting of a plurality of discrete regions, for example layers, inserts or coatings, each derived by compaction from a particulate composition. In a heterogenous tablet according to the present invention, each discrete region of the tablet will preferably have a mass of at least 5 gm.
In a heterogeneous tablet, at least one and possibly more of the discrete regions contains the mixed anionic and nonionic surfactants and detergency builder in accordance with the invention.